The increasing scarcity and the realization of ecological and safety problems associated with non-renewable energy reserves, such as coal, petroleum and uranium have made it apparent that it is essential that increased use be made of alternate non-depletable energy resources such as photovoltaic energy. Photovoltaic use has in the past been limited to special applications in part due to the high cost of manufacturing devices capable of producing photovoltaic energy. The development of a continuous process that continuously deposits successive layers of amorphous semiconductor alloy material on an elongated substrate to fabricate photovoltaic devices in mass production has greatly promoted the use of photovoltaic energy.
Recently, considerable efforts have been expended to develop systems and processes for preparing thin film amorphous semiconductor alloy materials which can be deposited so as to form p-type and n-type semiconductor alloy layers which can encompass relatively large areas for the production therefrom of thin film photovoltaic devices. It should be noted at this point that the term "amorphous" as used herein, is defined to include alloys or material exhibiting long range disorder, although said alloys or materials may exhibit short or intermediate range order or even contain crystalline inclusions.
In the economical continuous processing, a sheet of substrate material may be continuously advanced through a succession of operatively interconnected, environmentally protected deposition chambers, wherein each chamber is dedicated to the deposition of a specific layer of semiconductor alloy material onto the sheet or onto the previously deposited layer. For example, in making a solar cell of p-i-n type configuration, the first chamber is dedicated for depositing a p-type amorphous silicon semiconductor alloy, the second chamber is dedicated for the deposition of a layer of substantially intrinsic amorphous semiconductor alloy material and the third chamber is dedicated for depositing an n-type amorphous silicon semiconductor material. Because each deposited semiconductor alloy, and especially the intrinsic semiconductor alloy must be of high purity, the environment in the deposition chambers, particularly in the intrinsic deposition chamber, is isolated from the deposition constituents within the other chambers. The diffusion of the doping constituents from the p-type or n-type deposition chamber is halted and the contamination the intrinsic process gases in the intrinsic deposition chamber by dopant gases is prevented.
Thin film silicon and/or germanium alloy materials have found many applications from incorporation into calculators to large area grids for providing power to either homes, neighborhoods or even whole communities. The continuous deposition process has been used to produce many of these large area grids.
Transparent photovoltaic devices have been known for particular applications. These transparent devices are formed by placing extremely thin layers of semiconductor alloy material on a glass or transparent plastic or similar material with a first transparent electrode on each side of this semiconductor alloys to withdraw electrical current generated by the semiconductor alloys. These transparent electrodes in the past have been made from transparent conductive oxides. Alternatively, bus grids made from opaque material can also be used where the spaces between the bus grids provide for transparency through the photovoltaic device.
These transparent photovoltaic devices are often formed on a preformed and preshaped device, for example, a windshield or architectural plate glass which requires that the process of applying the thin film semiconductor layer on the substrate be a batch process with the substrate windshield etc. being moved from one deposition chamber to a sequential deposition chamber. The glass substrate needs to be appropriately masked beforehand. The opening of a deposition chamber to allow the movement of the preformed device to a sequential deposition chamber introduces the risk that contaminants may be in contact with the semiconductive alloy layers which can ruin the power generating aspect of the device. Furthermore, the batch process of depositing photovoltaic layers on a preformed transparent substrate is costly relative to the continuous process developed with thin sheets of material such as stainless steel.
There is a need for an optically transmissive photovoltaic device which incorporates a power generating sheet that is made with the known and cost-effective continuous deposition processes. Such devices would have wide application on car roofs and architectural glass where it is desired to have at least a portion of light that impinges on the photovoltaic devices to pass therethrough.